My foam squares are mounted in clusters of 8. I used cheap 1/4" masonite and cut 20in x 40in panels.
Foam is mounted to the panels with ARLENE'S Foamtastic glue (available at any sewing/crafts store for $4 a bottle.) It's like a thick white glue and you don't need much per square, just a couple of quick swipes. Best glue I've found; none of my panels have dropped in a year.
I used just 2 standard picture hanging wall inserts per panel at top and bottom center, but I suspect that one would do as each panel doesn't weigh much (I just wanted the the panel to be flush against the wall and not rattle.) Whenever I decide to move a little drywatch patch and quick dab of paint will be all that's needed.
The Arlene's glue is the real hint, guys. Super stuff....